Atherosclerosis is treated in arteries of the heart, head, neck and peripheral portions of the body using many different methods. The most popular methods, such as angioplasty, bare metal stenting, drug eluting stenting (permanently implantable and biodegradable), various types of energy delivery and rotational atherectomy, all treat an artery equally around the circumference of a target length of the arterial lumen. These devices are generally circumferentially symmetric, and cannot selectively treat one circumferential sector of the targeted length of the artery any different from another. Almost always, the targeted length of the artery identified for treatment is determined using angiography, which graphically depicts a vessel lumen, or intravascular ultrasound (IVUS), which graphically depicts the atherosclerotic plaque itself. With IVUS, the thickness of the atherosclerotic plaque can be determined along the length of the diseased area and at specific radial positions around its circumference. More often than not, the plaque is eccentric and thus varies in thickness at particular positions of a circumferential cross-sectional of the vessel. Treatment of plaque using the aforementioned circumferentially symmetric methods can sometimes cause undesired results. For example, drug eluting stents deliver drugs that inhibit neo-intimal proliferation (known as restenosis). In the section of artery where the stent is expanded, any normal (non-diseased) portion of vessel may not benefit from getting the same dosage of drug as the diseased portion.
Some methods for treating atherosclerosis, such as directional athrectomy, needle aided drug injection or certain types of brachytherapy (radiation), can actually vary the treatment along different circumferential sectors of the artery. The catheters used for these treatment methods are typically circumferentially asymmetric and have at least a portion that is torquable (rotatable), and thus able to be steered into a desired circumferential orientation. However, effective use of the asymmetric treatments is difficult because of certain characteristics of current imaging methods. For example, because angiography only shows an image of the lumen of the blood vessel, it is impossible to identify exactly where, in a particular circumferential cross-section, the atherosclerotic plaque is located and the plaque's thickness. IVUS does make it possible to view the circumferential location and thickness of atherosclerotic plaque in a length of a vessel, but unless the ultrasonic transducer is attached to the actual treatment device, it is difficult to use the IVUS image to direct the treatment catheter with precision. This is especially difficult in coronary arteries, where heart motion adds error. Attempts to include transducers on the treatment catheter have been moderately successful (U.S. Pat. No. 6,375,615 to Flaherty) but the additional components make it more difficult to build a small catheter, which is flexible and can track easily in the artery. Some other catheters have been developed (U.S. Pat. Nos. 4,821,731 and 5,592,939, both to Martinelli) which can combine IVUS imaging with tip positioning technology. This enables displaying a three dimensional graphical representation of the plaque, including any tortuosity inherent in the artery. However, additional capital equipment is required in the procedure room to perform this type of imaging and adds cost to performing the procedure.
Most of the methods described above are predominantly used to improve blood flow in stenosed areas of the artery, thus allowing for better delivery of blood to downstream tissue. Recently, more attention has been paid to vulnerable plaque—plaque that is prone to rupture, even though it may not actually be a stenotic lesion that limits flow prior to rupture. This is especially critical in coronary arteries, where a lesion rupture, combined with thrombosis, can cause a serious or even fatal myocardial infarction (heart attack). The lesion rupture can actually cause material, such as tissue factor, to dump out of the plaque, into the bloodstream, forcing the blood into a hypercoagulable state. Currently, angiography is of limited value in identifying vulnerable plaque, because this plaque is often non-stenotic, and looks similar to the normal vessel on an angiogram. New tissue characterization methods associated with IVUS (U.S. Pat. No. 6,200,268 to Vince and U.S. Pat. Nos. 6,381,350,7,074,188, and 7,359,554 to Klingensmith, as well as U.S. patent application Ser. No. 10/647,977, show promise for identifying vulnerable plaque, and a patient having a significant amount of vulnerable plaque. There are currently no standard methods to treat patients having vulnerable plaque once such patients are identified.